1. Field of the Invention
The present invention pertains to methods and materials for treating wounds whether accidentally caused (trauma, burns, etc.) or intentionally created by surgery. In particular, it pertains to the preparation of amnion membranes and methods for treating burns and wounds with this material.
2. Description of the Prior Art
Any injury or surgical procedure that leaves a portion of the body devoid of skin places the organism in dire jeopardy with respect to survival. Essential body fluids escape through large open wounds. Similarly, pathological organisms (fungi, bacteria, microplasms, etc.) easily enter the body through these same sites.
In most cases, wounds are closed by either: physically bringing the margins together vis-a-vis suture material (healing by primary intention) or by allowing the trauma site to heal naturally with new skin growing out and over the wound surface (healing by secondary intention). If the body is able to quickly close the wound opening before pathogens can invade and/or otherwise enter the tissues then the probability of survival is high. However, if wound closure is slow, then the chances of sepsis is high.
If a wound is so large as to prevent the approximation of its margins using sutures, then a dressing has to be used to temporarily seal off the wound from the surrounding environment. The ideal wound dressing should be thin, flexible and inert; provide adequate gas exchange; adhere tightly to the wound surface; prevent microbial invasion; surpress fluid loss; offer a template for new skin growth; should be readily available, easily stored, easily used, relatively cheap, and widely distributed.
The problem of providing a suitable wound dressing has existed since the time when the first open wounds were dressed or covered in order to facilitate healing. The intellectual search for the ideal wound dressing has probably been underway for the past 250 years. Its significance has been recognized for a similarly long period of time because nearly all large (greater than 40% total surface area) burn injuries led to death by bacterial sepsis prior to 1943. The chances for survival were largely improved when the hypochlorite bathing solution was first used (Bunyan-England 1940/1943) the advent of antibiotics and later when the pig skin temporary wound dressing was introduced.
Currently, Open wounds are:
(1) left uncovered and/or untreated; PA0 (2) covered with an anti-bacterial ointment (i.e. silversulphadiazine); PA0 (3) temporarily covered with pigskin; PA0 (4) temporarily covered with fresh amnion; PA0 (5) covered by homographic graft from other areas of the body; PA0 (6) covered with heterographic skin (from mother, father, brother, sister, etc.) PA0 (7) covered with a cloth or non-biological membrane or bandage; or PA0 (8) any combination of the above.
None of these are totally acceptable. Pigskin sloughs off after twenty to twenty-five days due to an antigen/antibody reaction. Fresh amnion can be enzymatically digested by the host in three to four days if the area covered is moist. Amnion may also evoke an antigen-antibody reaction and there is also a risk of transferring pathogens from donor to recipient. Homographic skin grafts cover one area while opening up another at the donor site. Heterographic skin is usually rejected by the patient in six to twenth-five days (another antigen/antibody reaction).
Because human amnion, being a continuation of the fetal integument, has many of the natural and physical qualities of skin and is relatively cost effective, it has become more popular as a temporary dressing than porcine xenografts in some burn treatment institutions. The successful use of fresh and/or frozen amnion as a biological dressing (i.e. on burn wounds), depends in part on its structure an integrity as a microbial barrier and on the sterility of the membrane, as well as the condition and nature of the wound to which it is applied.
Although amnion is a popular temporary dressing, in many ways it is even more temporary than pigskin or heterographic skin. Electronmicroscopy reveals the near total degradation of amnion by the host as early as three days post application if the covered site remains moist. The degradation appears to be due to digestion by enzymes in the wound exudate observable as early as twenty-four hours after application. Ultra-structural features of the amnion dermis, epidermis and basal lamina are completely lost after forty-eight hours on the patient. After seventy-two hours, the membrane components are difficult if not impossible to discern. Thus, the barrier qualities of the sterile fresh amnion may be lost long before the patient's need for barrier protection has ended. It is now recommended that, when used on severe wounds, the amnion be replaced at approximately three day intervals. In addition, electron microscopy discloses that some of the collagen filaments in the amnion dermis are partially unraveled during sterilization procedures (which usually empoloys a sodium hypochlorite solution) required to render the amnion asceptic which further compromises the barrier quality of the membrane even prior to its application to the wound.